1. Field of the Invention
The present invention relates to the field of power electronics. It concerns a high-power semiconductor module as claimed in the preamble of the first claim, in particular an IGBT (Insulated Gate Bipolar Transistor) module.
2. Discussion of Background
In the case of thyristors in which contact is made by pressure, for example, it has been found that a defect leads to a short circuit. With large chip areas, this short circuit remains stable over a long time. If redundant thyristors are provided in a stack of thryistors connected in series, then the remaining, intact thyristors withstand the voltage during the switched-off phase, and the stack remains operative. Defective thyristors can be replaced subsequently in the course of planned servicing work.
In a thyristor module, the semiconductor, that is to say the Si, is in mechanical and electrical contact between two Mo wafers. Si has a melting point of 1420xc2x0 C., that of the Mo is higher, and the intermetallic compounds of Si and Mo have an even higher melting point. Thus, in the event of a defect, the Si melts locally first of all and, as current flows, it forms a conductive channel composed of molten Si over the entire thickness of the semiconductor. This defect zone can propagate and/or move, but will only ever affect a small part of the chip area. In hermetically sealed housings, the molten Si does not oxidize but reacts with Mo to form a type of powder. This process continues until all the Si has been consumed, and may possibly extend over years.
In contrast to thyristor semiconductor components, IGBT chips are not produced as large-area units and, normally, a plurality of small-area individual chips are arranged isolated and alongside one another in the IGBT modules. Such a module is disclosed, for example, in EP 0 499 707 B1.
It is has now been found that no stable short circuits of the type described above can be expected with IGBT modules in which contact is made by pressure. This is primarily due to the reduced area of the individual chips, and the small silicon volume. The pseudo-stable phase of a short circuit lasts for only a few hours in this case. Furthermore, the housings are often deliberately not hermetically sealed, so that the molten silicon can react with oxygen and form insulating SiO2. Without any stable short-circuit path in the defective chip, the worst-case situation which can arise is as follows. If the remaining chips in a module, including the actuation, are still intact, they can withstand voltage during the switched-off phase. The current is then forced through the defective chip and, at voltages up to the break down voltage of the intact chips, can lead to a plasma being formed, with a very high power density. This results in the entire module being destroyed.
Accordingly, the object of the present invention is to provide a novel power semiconductor module which is formed from small-area individual chips and in which a short circuit of an individual chip does not lead to total failure of the module. In the case of a component of the type mentioned above, the object is achieved by the features of the first claim.
The essence of the invention is that a layer composed of a suitable material, for example, Ag, is brought into direct contact with one or both of the main electrodes of the Si semiconductor. The material of this layer must form a eutectic mixture with Si. In the event of a short circuit, the entire sandwich structure is heated and, once the melting point of the eutectic mixture is reached, a conductive melt starts to form on the contact surface between the said layer and the Si. This zone can then expand over the entire thickness of the semiconductor, and thus form a metallically conductive channel.
According to the invention, in the event of damage, a stable short circuit is facilitated by a metallically conductive channel being formed between the main electrodes of the affected Si semiconductor chip. This channel is limited to one part of the chip area, but carries the entire rated current, and thus prevents further heating of the rest of the Si. The melting point of the metallically conductive melt in this channel, or of the corresponding solid compound containing silicon and silver, must thus necessarily be below the melting point of pure Si.